LED, hidden from view, mounted inside pen to illuminate for writing are already disclosed in international patent database.
Pressing a pushbutton, pushing a slide, rotating a shaft or a cap, or elevating a refill etc., may electrify LED inside pen to illuminate and a further press, push, or rotation of a switch darken it.
As above, equipped with an on-off switch to determine whether LED is lit simply manually operated by user. But, LED inside pen lit up spontaneously without manual operation without on-off switch is sought by the present invention.